In a typical case such as a transmission drive chain, the degree of tension or slack in the chain is adjustable by a person un-clamping a slipper member, moving the slipper member to a new location where the chain is tighter, and then re-clamping the slipper member. This adjustment is not automatic, in that the chain gets progressively slacker until the person effects the adjustment.
In other known types of tensioner, a spring presses the slipper against the chain, whereby tension is maintained in the chain as the chain wears, due to the resilience of the spring.
In a case where the chain acts uni-directionally, i.e only in forward-drive, a spring-biassed slipper pressed against the slack-run of the chain can be adequate. But in the case where the chain acts sometimes in forward-drive, and sometimes in reverse-drive, a spring-biassed slipper pressed against the slack run of the chain is not enough, because the slack-run becomes the tight-run in reverse.
Providing two slippers, one to each run, and spring-biassing them together by means of a floating spring clamp, can serve in those cases. However, in that case it is hard to achieve the right compromise of spring forces over the required range of movement: particularly since sudden reversals of load can hurl the tensioner suddenly from side to side.